The present invention relates, in general, to a fluid pressure responsive multiple-circuit system for use on a motor vehicle, which system includes at least one operating service brake circuit and a parking brake circuit. More particularly, the invention relates to a method of and apparatus for automatically controlling the parking brake circuit should a defect occur in the operating service brake circuit.
Presently known fluid pressure responsive multiple-circuit systems for use on motor vehicles, and particularly trucks, typically comprise a plurality of service brake circuits, a parking brake circuit, and usually at least one auxiliary circuit for various accessories of the system. The parking brakes on these vehicles are usually spring-applied and released by an application of fluid pressure sufficient to overcome the spring loading. In these systems, which usually operate with pneumatic pressure, safety or overflow valve devices have been used to control the charging of the several circuits from a common fluid pressure source. Each of such circuits may be charged through a respective safety valve in a certain desired order such that, when charging, fluid pressure is supplied to the auxiliary circuits only after the service brake circuits have been charged. This priority charging of the circuits then ensures that the vehicle may be set in motion only after the pressure has been adequately built up in at least one of the service brake circuits and in the parking brake circuit at least sufficient to release the spring-loaded cylinders applying the parking brakes.
Overflow valves are customarily designed to open when a sufficiently high pressure has built up on their inflow side; this is known as the valves's opening pressure. For a number of reasons, this opening pressure is, as a rule, not constant for any particular overflow valve. One of the primary reasons for this is because initially the pressure on the outflow side of the overflow valve is lower than the pressure on the inflow side; and, in the normal case, the opening pressure required will decrease with an increasing pressure on the outflow side.
This effect is related to such customary design of an overflow valve. In this design, a valve element, such as, a valve piston, is pressurized in the closing direction by an adjusting force, for example, a spring; and in the opening direction both by the pressure on the inflow side and, in addition, by the pressure on the outflow side of the valve. At higher pressures on the outflow side of the valve, a lower opening pressure is required to initially open the overflow valve, and vice-versa. Consequently, the opening pressure is at its highest if only atmospheric pressure prevails on the outflow side (or in a more theoretical case, a less than atmospheric or even a negative pressure). The opening pressure, therefore, lies within an opening pressure range.
Once the respective consumer circuit overflow valve is opened, then, except for any flow losses which may occur over the cross-section of the valve, there is essentially an equalization of pressure between the inflow side and the outflow side of the overflow valve, i.e., in the present case, between the supply chamber and the associated consumer circuit, or the operating brake circuit and another consumer circuit, such as, the parking brake circuit.
If, when the overflow valve is open, the essentially identical pressure in the supply chamber and the associated consumer circuit cannot be maintained, for example, because of a leak in that particular consumer circuit, then this pressure will drop. If the pressure drops to a predetermined value, which value is the closing pressure for the particular overflow valve, then the overflow valve closes and cuts off the consumer circuit associated with that particular overflow valve from the supply chamber. Thereafter, the pressure in that consumer circuit can fall further; while in the supply chamber, the closing pressure is maintained without any recharging by the compressed air system. In the case of a recharging by the compressed air system, the air pressure in the supply chamber rises again until it reaches the nominal pressure established for the compressed air installation.
If the essentially identical pressure in the supply chamber and the consumer circuit cannot be maintained on the inflow side as the result of a leak in the vicinity of the supply chamber, for example, at the inflow supply chamber, the overflow valve acts accordingly, i.e., after its closing, the pressure in the supply chamber can drop further while the pressure in the consumer circuit--assuming there is no air consumption--remains at the level of the closing pressure.
The closing pressure--like the opening pressure--is measured on the inflow side, i.e., in the supply chamber. As a result of the customary design of an overflow valve, as described above, its closing pressure is always below its opening pressure range, whereby the difference between the maximum opening pressure and the closing pressure is essentially determined by the ratio of the surface areas on the inflow side and the outflow side of the control element for a particular overflow valve.
This opening and closing behavior of an overflow valve, described above, is put to use in protection systems of the types described above for a mutual protection of the individual consumer circuits.
For an understanding of one of the disadvantages of such prior art systems, assume that the overflow valves of all the consumer circuits are opened, and that essentially there exists a pressure equalization between the supply chamber and each of the consumer circuits. Next, assume that, in one or more of the consumer circuits, there occurs, for some reason, an undesirable outflow of compressed air into the atmosphere, which is large enough to cause a drop in pressure in the supply chamber. This pressure drop is then propagated by the open overflow valves in all the consumer circuits.
Such pressure loss can be caused by a leak, in which case, the consumer circuit in question is actually defective, or by a minor lack of sealing in one or more of the connections required for the consumer circuit in question. These causes of the loss of pressure will be described below by the terms "defect" or "leak".
It is apparent that to cause a drop in the pressure, the leak must be large enough that all of the flow being supplied by the compressed air installation escapes. In other words: when the flow equals zero, or when there are small flows, even small leaks are sufficient; and with larger flows, correspondingly larger leaks are necessary.
This drop in pressure continues in the defective consumer circuit, or possibly circuits, until the closing pressure of the overflow valve or valves associated with the defective consumer circuit or circuits is reached. Then these overflow valves close and cut off the defective consumer circuit or circuits from the supply chamber. In this manner, the pressure loss in the supply chamber and in the other consumer circuits--if their overflow valves are still open--comes to an end. If one or more of the other overflow valves, for example, because of manufacturing tolerances has a higher closing pressure, then this valve or these valves will have already closed and the corresponding consumer circuit will have been cut off from the supply chamber and the pressure drop ceases in the defective consumer circuit or circuits.
In the case of a defect in at least one consumer circuit, therefore, the other consumer circuits are protected by at least the value of the closing pressure of the overflow valve corresponding to the defective circuit. This closing pressure can, therefore, be called the "protected pressure".
If the compressed air system now supplies air, then the pressure in the intact consumer circuits will increase to the opening pressure of the overflow valve of the defective consumer circuit. This is its maximum opening pressure, if the pressure in the defective consumer circuit has already dropped to atmospheric pressure.
Protection systems, as described above, therefore, make certain that in the case of a defect in one or more consumer circuits, the other (intact) consumer circuits, except for a temporary drop in pressure to the "protected pressure", remain fully-operable up to the specified opening pressure.
But, for example, because of the above-mentioned manufacturing tolerances, each of the overflow valves in a protection system in corresponding consumer circuits can have different opening pressures. Such varying opening pressures of these overflow valves can cause problems in certain cases. A typical case of this sort exists if all the consumer circuits exhibit atmospheric pressure if a consumer circuit is defective, and if the overflow valve corresponding to this defective circuit exhibits the lowest opening pressure. Then, if the defect is large and pneumatic pressure is being supplied by the compressed air installation, it can happen that the entire flow will flow through the overflow valve of the defective consumer circuit into the defective circuit. In this situation, the compressed air flows out through the leak into the atmosphere without the other overflow valves of their respective consumer circuits opening. Therefore, these circuits do not become operable. The reason for this is that they do not experience an increase in pressure.
One system developed to overcome this problem is taught in my U.S. Pat. No. 4,057,298, the teaching of which is incorporated herein by reference. In this system, the supply chamber is connected with a consumer circuit parallel to its overflow valve by a throttle channel which includes a check valve disposed therein which cuts off the air flow from the consumer circuit to the supply chamber. The throttle channel is thereby designed so that, if the consumer circuit is defective, the quantity of air flowing through it into the consumer circuit does not suffice to significantly overcome (or to overcome at all) the atmospheric pressure prevailing in the defective consumer circuit and to endanger the operating safety of the remaining intact consumer circuits. In the intact circuits, on the other hand, because of the quantity of air flowing through the throttle channel, a pressure builds up in such intact circuits which is sufficient to lower the opening pressure of the overflow valve to a point where it will open with some degree of certainty. The task of the above-mentioned check valve is to prevent any pressure loss of an intact circuit via the throttle channel and from the supply chamber into a defective consumer circuit.
On an overflow valve which--like the one described above can be bypassed by a throttle channel, the reduced opening pressure, which is set with a consumer circuit initially emptied to atmospheric pressure, is called the "nominal opening pressure".
The difference between the "nominal opening pressure" and the closing pressure is also a function of the ratio of the surface areas on the inflow side and the outflow side of the element of the overflow valve.
A requirement exists, however, that upon the drop of air pressure in the operating brake circuit or circuits to a predetermined minimum pressure (e.g. 40 psi), the parking brakes must automatically be engaged, and such parking brakes should not be capable of being released before this minimum pressure has again increased up to such minimum pressure.
This requirement can be met on a motor vehicle pneumatic brake system equipped with a protection system of the type described, if the "protection pressure" of the operating brake circuits and the closing pressure of the overflow valve of the parking brake circuit are below the above-mentioned minimum pressure. Such a low protection pressure and closing pressure, however, is unacceptable in terms of the above-mentioned parking brake task.
On the other hand, with a higher "protection pressure" and closing pressure, the above-mentioned requirement for the automatic engagement of the parking brakes on a motor vehicle pneumatic brake system, equipped with a protection system of the type described, cannot be met. Accordingly, the parking brake circuit (like the other consumer circuits) is cut off from the defective operating brake circuit or circuits. This cut-off occurs at the "protection pressure" of the overflow valve or valves of the operating brake circuit or circuits, or at the closing pressure of the overflow valve associated with the parking brake circuit. The pressure in the parking brake circuit, therefore, does not participate in the drop of air pressure in the questionable operating brake circuit or circuits below the above-mentioned minimum pressure, and the parking brakes are not evacuated.
Whether the portion of the above-mentioned requirement regarding the prevention of the release of the spring-applied parking brakes as long as the operating brake circuit or circuits have not been pressurized to the above-mentioned minimum pressure can be met--depends on the pressure in the parking brake circuit. This parking brake circuit pressure may, for example, be the residual pressure from an earlier operation of the motor vehicle. If the residual air pressure is higher than the above-mentioned minimum pressure, the requirement is not met; if it is lower, the requirement is met. Therefore, such prior art valve arrangements do not provide the desired functions.